The invention relates generally to porous solid high explosives, and more particularly to the stabilization of porous solid high explosives.
A high explosive is a porous material, with a wide distribution of pore sizes. The pore sizes typically range from about 0.01 .mu.m to sizes exceeding 10 .mu.m, with a multi-mode distribution that reflects the process by which the explosive was prepared. Based upon examination of electron micrographs of undetonated explosives such as TATB and HMX, it appears that mean pore size is less than 0.1 .mu.m, with perhaps one percent by volume of pore sizes being about 10 .mu.m and one percent by volume being about 50 .mu.m. These larger size pores (diameter greater than or about 10 .mu.m) are initiation sites at shock pressures of the order of 30 Kbar. Ignition occurs in the material surrounding one of these large pores if the heat content of a hot spot (where the internal energy from a passing shock wave is locally much higher than in the adjacent material) reaches a threshold value for a runaway reaction.
Over the last eight years the theory of initiation of runaway reaction in explosive materials has centered around the existence of these pores. The current understanding is summarized in a review of explosive models: Gerald L. Nutt, "A Reactive Flow Model For a Monomolecular High Explosive", J. Appl. Phys., 64 (4) August 1988, particularly Section II.
The primary mechanism involved in the shock initiation of solid high explosives is the visco-plastic heating of the explosive material surrounding the microscopic pores left in the material during manufacture. The initiating shock causes the pores to collapse. The resulting local heating can raise the temperature to the critical value required for runaway reaction in the explosive. Whether or not the temperature reaches the critical value depends on the values of the pressure, the pore volume and the heat conductivity of the explosive material, among many other less significant parameters.
The presence of the larger pores, and the consequent instability of solid high explosives creates a danger of ignition and detonation, particularly when the explosives are transported or stored. Therefore, it would be desirable to provide a stabilized explosive, and method for making same, to reduce the danger of ignition and detonation during transport or storage. The largest pores cause the greatest instability, e.g. pores of about 10 .mu.m diameter or greater. However, the pore size cannot generally be reduced or eliminated during the conventional manufacture of the explosives. There is no practical way to prevent the formation of some pores with diameters of about 10 .mu.m or greater during the manufacturing process. Therefore, all conventionally manufactured explosives will contain sufficiently large pores that are susceptible to shock initiated detonation at relatively low pressures.
There have been previous efforts to desensitize solid high explosives, such as mixing an energetic, relatively sensitive, explosive with one much less sensitive and somewhat less energetic. An example is the LLNL developed RX-26-AF. This explosive was an approximately equal mixture of HMX and TATB. The hope was that the mixture would have the sensitivity of the least sensitive component of the mixture (TATB) and performance approaching the more energetic component (HMX). This attempt was unsuccessful as sensitivity to shock initiation was found to be determined by the most sensitive component of the mixture, HMX.
On the other hand, there are processes which successfully make insensitive explosives more sensitive. Some commercial slurry explosives are extremely insensitive which allows them to be safely transported through populated areas. When the explosive is emplaced, it is sensitized by mixing in tiny glass microspheres.